Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Topological zeolite structure are described in Atlas of Zeolite Framework Types, which is maintained by the International Zeolite Association Structure Commission at http://www.iza-structure.org/databases/. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
Catalytic cracking processes are used in several areas of the refinery. Fluidized catalytic cracking (FCC) converts heavy feeds to lighter products including diesel, gasoline and light olefins. FCC was traditionally used for conversion of heavy feeds such as VGO (vacuum gas oil) to gasoline and diesel but has recently been extended to the co-production of propylene. This process typically uses at least one of a 12-membered-ring and or a 10-membered-ring zeolite such as FAU or MFI to catalyze the conversion of heavy feeds with high selectivity to gasoline and/or propylene. Naphtha cracking has been heavily studied and converts naphtha feeds with high selectivity to propylene using 12-membered ring or 10-membered ring zeolites. Olefin cracking converts olefinic feeds such as butenes or pentenes with high selectivity to propylene using 10-membered-ring zeolite catalysts such as MFI and MEL. In all these catalytic cracking processes, new catalysts are continuously needed with high overall conversion of the feedstock and good selectivity to propylene.
Especially advantageous would be a commercially utilizable catalyst containing 12-membered rings and 10-membered rings in the same 3-dimensional structure. Commercial utility is typically seen in aluminosilicate structures which are synthesized in hydroxide media with readily available structure directing agents. Zeolites which contain both 12-membered and 10-membered rings in 3-dimensional structures belong to the CON, DFO, IWR, IWW and MSE structure types. The synthesis of CIT-1, a zeolite of the CON structure type, is described in U.S. Pat. No. 5,512,267 and in J. Am. Chem. Soc. 1995, 117, 3766-79 as a borosilicate form. After synthesis, a subsequent step can be undertaken to allow substitution of Al for B. The zeolites SSZ-26 and SSZ-33, also of the CON structure type are described in U.S. Pat. No. 4,910,006 and U.S. Pat. No. 4,963,337 respectively. SSZ-33 is also described as a borosilicate. All 3 members of the CON structure type use very complicated, difficult to synthesize structure directing agents which make commercial utilization difficult. The known member of the DFO structure type is DAF-1 which is described as an aluminophosphate in Chem. Commun. 1993, 633-35 and in Chem. Mater. 1999, 11, 158-63. Zeolites from the IWR and IWW structure types are synthesized only in hydrofluoric acid containing synthesis routes, making commercial utilization difficult.
One particular zeolite of the MSE structure type, designated MCM-68, was disclosed by Calabro et al. in 1999 (U.S. Pat. No. 6,049,018). This patent describes the synthesis of MCM-68 from dication directing agents, N,N,N′,N′-tetraalkylbicyclo[2.2.2]oct-7-ene-2R,3S:5R,6S-dipyrrolidinium dication, and N,N,N′,N′-tetraalkylbicyclo[2.2.2]octane-2R,3S:5R,6S-dipyrrolidinium dication. MCM-68 was found to have at least one channel system in which each channel is defined by a 12-membered ring of tetrahedrally coordinated atoms and at least two further independent channel systems in which each channel is defined by a 10-membered ring of tetrahedrally coordinated atoms wherein the number of unique 10-membered ring channels is twice the number of 12-membered ring channels.
Applicants have successfully prepared a new family of materials designated UZM-35. The topology of the materials is similar to that observed for MCM-68. The materials are prepared via the use of simple, commercially available structure directing agents, such as dimethyldipropylammonium hydroxide, in concert with small amounts of K+ and Na+ together using the Charge Density Mismatch Approach to zeolite synthesis (U.S. Pat. No. 7,578,993).
The UZM-35 family of materials is able to provide and maintain high conversion during catalytic cracking reactions and high selectivity to propylene due to its particular pore geometry and framework Si/Al ratio. The UZM-35 material contains significant amounts of Al in the tetrahedral framework, with the mole ratio of Si/Al ranging from about 2 to about 12. Al content in the framework is known to correspond to high activity in catalytic cracking processes.
Due to the unique structure of UZM-35, catalysts made from UZM-35 are able to show high conversion of n-heptane with good selectivity to olefins. In n-heptane cracking, UZM-35 converts greater than 70% of n-heptane at 500° C. with high selectivity of 34% to propylene with only 12% selectivity to light alkanes (C1, C2, C3). In comparison, with the same mass of zeolite, DHCD-4-CB (MFI based catalyst) converts 84% of the n-heptane feed with a selectivity to propylene of only 30%. Additionally, DHCD-4 has a selectivity of 15% to light alkanes under these conditions.
In the conversion of C4-olefins to propylene by catalytic cracking, UZM-35 catalysts are also effective. UZM-35 gives much higher conversion than the reference MFI catalyst with similar light olefin yields at equivalent conversion. For the reference MFI, a conversion of 55 wt-% is achieved at 580° C., 7 psig, 13.5 WHSV with selectivity of 65 wt-% to propylene. For UZM-35, 52 wt-% conversion is achieved at 520° C., 7 psig, 40 WHSV with a propylene selectivity of 63.5 wt-%. UZM-35 gives the beneficial property of running at much less harsh conditions for the same approximate light olefin yields.